How to fade out a mesh?
In the materials tab of your mesh, add a new material if there is not one already. Then, scroll down until you find the transparency tab. Enable transparency, and you will see a slider that allows you to change the opacity (or transparency) of the material, and thus, the mesh. I would use z-transparency as it is the most realistic.

With the opacity set to full, choose the frame when you want to begin the fade, put your mouse over the opacity slider I. Then, move to the frame when you want the mesh to be transparent, slide the opacity slider to 0 and hit I (with your mouse over the slider) again.

This technique is for blender internal